Interpretive Summary: Vegetable growers in the southeastern United States usually produce collard and kale during the cooler months of the year. However, to extend their annual production, growers are trying to produce these crops during hot summer months. Summer production of collard and kale can be successful, but summer plantings are increasingly infected with a hot weather disease called yellows, caused by a fungus. As a consequence of this disease, producers are suffering economic losses in summer plantings. Cooperative research between Clemson University and ARS has shown that different varieties of collard respond differently when grown during summer in fields containing a high level of the yellows fungus. Some varieties are resistant to yellows, show no signs of the disease, and produce normal yields. Other varieties are susceptible, become infected, and are destroyed by the disease. All varieties of kale tested were very susceptible to yellows. Additional controlled temperature studies confirmed results of field trials and indicated that the collard or kale variety grown is much more important in determining the level of disease that might occur than is the actual temperature at which varieties are grown. This research provides vegetable producers with valuable information, illustrating that collard can be grown during summer without becoming infected with yellows, as long as producers choose varieties resistant to the disease. On the other hand, it shows producers take a great risk in growing kale during summer since all varieties are susceptible. This research is also valuable to collard breeders because it identifies yellows resistant varieties that might be used in future collard improvement efforts.

Technical Abstract:
The cole crop disease yellows, caused by Fusarium oxysporum f. sp. conglutinans, can be very damaging to collard. Southeastern growers are more frequently producing collard in hot, summer months when conditions for yellows development are favorable, and thus, incidence of this disease is likewise increasing. A collection of essentially all U.S. commercial collard cultivars, various collard landraces and other cole crop representatives was evaluated for their response to artificial inoculation with F. oxysporum f. sp. conglutinans under controlled-temperature conditions. In addition, the same group of entries was evaluated for response following transplanting to naturally-infested soil in the field during the summer of 1997 and 1998. In all trials, entry had the most significant effect on percent diseased or dead plants. Temperature was important in one growth chamber experiment with five entries but was not significant in a second experiment with 20 entries. In the field, although significant differences were observed among entries and between years, a significant entry X year interaction was not detected. There was a significant correlation between results from controlled-environment studies and results from the field. A high level of resistance to F. oxysporum f. sp. conglutinans was expressed in particular collard cultivars including 'Flash', 'Heavicrop', and 'Morris Heading', and also in particular landraces. Choice of collard cultivar is a critical factor in trying to produce a crop where conditions favor F. oxysporum f. sp. conglutinans infection. Results of this research will aid development of new yellows- resistant cultivars.